Myocardial ischemia is the leading cause of morbidity and mortality in developed countries. Myocardial ischemia involves oxygen starvation of the myocardium, particularly in the bulky left ventricular wall, that can lead to myocardial infarction and/or the onset of malignant arrhythmias if the oxygen starvation is not alleviated. Although myocardial ischemia is associated with the symptom of angina pectoris, the majority of episodes of myocardial ischemia are asymptomatic or "silent."
Accurate and rapid detection of myocardial ischemia is the first essential step toward reducing morbidity and mortality from this often silent but deadly condition. Without the knowledge of the condition, it cannot be treated. A wide range of therapies are known for the treatment of myocardial ischemia once it is detected, including surgical revascularization, neural stimulation and a variety of biologically active agents or compounds which can remove blood clots, reduce cardiac workload or improve cardiac circulation.
The electrocardiogram (ECG) or electrogram (EGM) of the cardiac cycle detected across sense electrode pairs located on the patient's skin or in the patient's body, respectively, is a repetitive waveform characterized by a periodic PQRST electrical activation sequence of the upper and lower heart chambers. The PQRST sequence is associated with the sequential depolarization and contraction of the atria followed by the depolarization and contraction of the ventricles, and successive PQRST complexes are separated by a baseline or isoelectric region. The PQRST electrical activation sequence commences with the P-wave indicative of the depolarization and contraction of the atria and is followed by the QRS complex indicative of the depolarization and contraction of the ventricles. The T-wave at the termination of the ST segment time delay is associated with re-polarization of the ventricles. The PQRST electrical activation sequence with intact A-V activation detected across a sense electrode pair is fairly predictable in shape. The P-wave, R-wave and T-wave events occurring in sequence in the range of normal heart rates are usually readily recognized by visual examination of the external ECG or an EGM recorded by implanted electrodes that are correctly oriented with the depolarization waves. The P-wave and R-wave are readily sensed by sense amplifiers of a monitor or therapy delivery device coupled with appropriately placed sense electrode pairs.
The ST segment of the ECG or EGM is typically close in amplitude to the baseline or isoelectric amplitude of the signal sensed between PQRST sequences, depending on the sense electrode pair location. During episodes of myocardial ischemia, the ST segment amplitude is elevated or depressed (depending on positioning of the ECG or EGM sense electrodes in relation to the heart) from baseline. These ST segment deviations can be readily recognized by visual examination.
The physiological basis of ST segment deviation changes in the presence of cardiac ischemia may be explained by ischemic changes in the action potential of cardiac myocytes. When myocytes become ischemic, the resting potential increases (toward zero), the depolarization slope of the action potential decreases, the plateau decreases in voltage, and the duration of the action potential decreases. These changes result in voltage gradients and an "injury current" between normal and ischemic myocardium during the resting and plateau phases of the action potential. Because the voltage gradient between the normal and ischemic myocardium is positive during diastole and negative during systole, the isoelectric or baseline signal level and the ST segment signal level of the ECG are displaced in opposite directions during ischemia. The change in the isoelectric or baseline level is not easily detected because the pair of sense electrodes implanted in the patient's body are AC coupled through filters to the inputs of differential sense amplifiers. However, the disparity between the isoelectric or baseline level and the ST segment may be detected if the isoelectric or baseline point and the ST segment point can be identified.
It has long been a goal in the development of external cardiac monitors and IMDs to be able to automatically detect ST segment deviations from baseline and to accurately determine when the heart is ischemic therefrom so that the patient's cardiac condition can be assessed and treated both in the clinical setting and while the patient is outside a clinical setting. A wide number of implantable therapy delivery devices and/or monitors have been proposed for detecting ischemia and delivering a therapy and/or recording the detected ischemic events in an ambulatory patient. Fundamentally, the algorithms employed in these systems endeavor to automatically sample the amplitude of the ST segment in the PQRST complex in an EGM or ECG signal, compare its absolute amplitude against a threshold and declare an ischemic or normal condition based on the results of the comparison.
In regard to Implantable Medical Devices (IMDs), commonly assigned U.S. Pat. Nos. 5,199,428 and 5,330,507 and U.S. Pat. No. 5,203,326, are incorporated herein by reference, and describe the historical development of electrical stimulation of the carotid and vagus nerves and other nerves to relieve cardiac arrhythmias and angina pectoris associated with myocardial ischemia. Perhaps more important to the background of this invention, they also describe relatively simplistic methods for detecting cardiac ischemia. The '326 patent also proposes providing backup anti-tachyarrhythmia pacing and cardioversion/defibrillation shock therapies. U.S. Pat. Nos. 5,531,768, 5,497,780, 5,135,004 and 5,313,953, all incorporated herein by this reference, monitor or detect myocardial ischemia and some record data related to ischemic episodes for telemetry out at a later time, to provide therapy or even to set off an alarm.
In these ischemia detection IMDs, the ischemia detection depends entirely or at least in part on the location of a fiducial point in the PQRST sequence, sampling the EGM signal level at a point within the ST segment in the PQRST sequence, and detection an elevated or depressed ST level exceeding a threshold level. Automatic detection techniques are set forth in the above-incorporated '428 and '507 patents that depend on sensing the R-wave, setting an ST segment time window timed from the detected R-wave, sampling the amplitude and/or integrating the amplitude to develop a current event ST signal level, and comparing the current event ST signal level to a threshold signal level that is derived from an average normal ST signal level. In the '953 patent, a computationally expensive template establishing and matching algorithm is set forth that determines "I" and "j" deflection points preceding and following the R-wave of each PQRST sequence as the fiducial point or points. The ST segment signal level is sampled 80 ms after the determined "j" point and is compared to the threshold signal level.
In the above-incorporated '428 patent, it was proposed that the detection of myocardial ischemia be accomplished by also sensing the patient's coronary sinus blood pH and/or oxygen saturation and comparing each to preset, normal thresholds. The sensors are located in the coronary sinus or a coronary vein to measure the dissolved oxygen and/or the lactic acid level of myocardial venous return blood. The system includes programmable thresholds against which the signals developed by the sensors and the ST segment deviation are compared. When ischemia is confirmed, the disclosed system triggered burst stimulation of selected nerves until the blood gas and/or ST segment variations returned to non-clinical risk levels. However, blood oxygen sensors that perform adequately over a period of chronic implantation have not been perfected, and blood oxygen changes can be due to conditions or physiologic states of the patient other than ischemia.
These prior approaches are also problematic for a number of reasons that contribute to the magnification of the deviation of the sampled ST signal level from the isoelectric level due to factors and conditions other than myocardial ischemia, thus registering too many false positive indications of ischemia to be very useful. Myocardial ischemia can be mistakenly detected due to ST segment changes in the PQRST complex caused by "axis shifts", electrical noise, cardiac pacing, and high sinus or tachycardia cardiac rates that distort the shape of the PQRST complex. These problems are described, for example, in "Analysis of Transient ST Segment Changes During Ambulatory Monitoring" by Franc Jager et al. at Computers in Cardiology, 1991, Los Alamitos: (IEEE Computer Society Press 1991; 453-456), "An Approach to Intelligent Ischemia Monitoring" by Bosniak et al. in Med. and Bio. Eng. & Comp, 1995, pp. 749-756, and in "A Compact, Microprocessor-Based ST-Segment Analyzer for the Operating Room" by Seven J. Weisner et al., (IEEE Trans. on Biomedical Engineering BME-29, No. 9:642-648.
For detection of axis shifts and eliminating their confounding effects on attempts to establish a reliable ischemia detection system, the Jager algorithm (from his article listed in the preceding paragraph) measures the electrical axis angle and the difference between the ST segment and the isoelectric level over two periods, one immediately after the other, and compares the difference in mean the parameters between these two periods to a threshold. Bosniak et al. use a multistate Kalman filter to look for step changes in ST segment, representing axis shifts. This method is far too complex for current generation implantable devices.
There remains a need for a system capable of automatically and reliably detecting ischemia. Significant advantage can be had if it is able to detect ischemia in any portion of the patient's heart. Ease of implantation, stability and long term use in ambulatory patients is obviously a consideration. Important also is that such a system reliably and consistently distinguish ischemia from other conditions or physiologic states of the patient. Additionally an indication of the location of the ischemia is useful too.
This can be characterized as a need for such a system for accurately detecting myocardial ischemia through measurements of the cardiac EGM in more than one sensing axis to account for the possible locations of ischemic regions of the heart that is easily implanted and functions reliably over time, even as the heart condition changes.